Conventionally, metallurgical slag is either granulated in water or cooled in slag pits. Water quenching ensures fast solidification of the metallurgical slag, which, in the case of blast furnace slag, is a necessary condition for obtaining a valuable product. The water is firstly used to fragmentize the slag stream into small particles and then to withdraw the energy through direct contact. As this has to happen at ambient pressure, the temperature of the slag is immediately lowered to a temperature level of below 100° C., which makes it impossible to recover energy in an efficient way. Cooling of metallurgical slag in slag pits needs longer cooling times and may give different product qualities. The thermal energy in the hot slag is consequently lost to the environment.
JP 2005306656 (A) describes a method for solidifying molten slag, by which a sound slag ingot, free from bubbles or solidification/shrinkage holes can be obtained by a simple method, and it is possible to effectively use the slag as an artificial block-shaped stone material. When slag is solidified by injecting molten reformed slag into a casting mold, the slag is rapidly solidified in the casting mold by charging oxide particles continuously or intermittently to the flow of the injected slag. Reformed slag or crushed solidified slag obtained by crushing a portion of the solidified slag produced by this method is preferably used as the oxide particles.
U.S. Pat. No. 4,359,434 discloses a process for granulating blast furnace slag melts, the melt being shaped into at least one thin, liquid melt stream moving freely in a predetermined direction and which, by meeting at a predetermined incidence angle with a stream of fine grained, solid particles flowing substantially freely in a substantially uniform direction at a high rate of flow in relation to the melt stream, being converted at least partly into a substantially fine-grained granulate having a fan-shaped distribution over at least part of the opposite angle to the incidence angle.
For a continuously operated blast furnace in the iron and steel industry at a theoretical average slag flow rate of 2 t/min, the thermal power contained in the slag is equal to 56 MW (thermal power=energy content (1200 J/kg/K)*temperature difference (1400 K)*flow rate (2 t/min=33.3 kg/s)=56 MW). This results in an electrical power of 22 MW if converted with an efficiency of 40%.
To use this potential in an efficient way, it is necessary to rapidly cool down the slag to a temperature level, which is low enough to make the treatment of the material easier but high enough to preserve the energy at a useable level. Care must also be taken to lower the temperature quick enough and far enough to obtain a vitrified slag rather than an amorphous slag, which is a lot less priced (about 15 times) in the market.
This can be achieved by mixing liquid slag with cold slag granulates of the same chemistry. The slag can then be subject to heat recovery in a heat exchanger.
It has been found however that due to the high viscosity of the liquid slag, the cold slag granulates and the liquid slag do not mix easily and thus it is not possible to cool the liquid slag fast enough to obtain vitrified slag.